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Four Ca2+ ions activate TRPM2 channels by binding in deep crevices near the pore but intracellularly of the gate.

Csanády L
,
Törocsik B
.

AbstractTRPM2 is a tetrameric Ca(2+)-permeable channel involved in immunocyte respiratory burst and in postischaemic neuronal death. In whole cells, TRPM2 activity requires intracellular ADP ribose (ADPR) and intra- or extracellular Ca(2+), but the mechanism and the binding sites for Ca(2+) activation remain unknown. Here we study TRPM2 gating in inside-out patches while directly controlling intracellular ligand concentrations. Concentration jump experiments at various voltages and Ca(2+) dependence of steady-state single-channel gating kinetics provide unprecedented insight into the molecular mechanism of Ca(2+) activation. In patches excised from Xenopus laevis oocytes expressing human TRPM2, coapplication of intracellular ADPR and Ca(2+) activated approximately 50-pS nonselective cation channels; K(1/2) for ADPR was approximately 1 microM at saturating Ca(2+). Intracellular Ca(2+) dependence of TRPM2 steady-state opening and closing rates (at saturating [ADPR] and low extracellular Ca(2+)) reveals that Ca(2+) activation is a consequence of tighter binding of Ca(2+) in the open rather than in the closed channel conformation. Four Ca(2+) ions activate TRPM2 with a Monod-Wymann-Changeux mechanism: each binding event increases the open-closed equilibrium constant approximately 33-fold, producing altogether 10(6)-fold activation. Experiments in the presence of 1 mM of free Ca(2+) on the extracellular side clearly show that closed channels do not sense extracellular Ca(2+), but once channels have opened Ca(2+) entering passively through the pore slows channel closure by keeping the "activating sites" saturated, despite rapid continuous Ca(2+)-free wash of the intracellular channel surface. This effect of extracellular Ca(2+) on gating is gradually lost at progressively depolarized membrane potentials, where the driving force for Ca(2+) influx is diminished. Thus, the activating sites lie intracellularly from the gate, but in a shielded crevice near the pore entrance. Our results suggest that in intact cells that contain micromolar ADPR a single brief puff of Ca(2+) likely triggers prolonged, self-sustained TRPM2 activity.

Figure 2. The rundown of TRPM2 currents in excised patches reflects a progressive decline in the number of active channels. (A) Macroscopic TRPM2 current (I; black trace) activated in an inside-out patch by exposure to 32 µM ADPR and 125 µM Ca2+ (bars). Red line is a single-exponential fit to the time course of current rundown, which was subtracted from the current trace to obtain the time course of gating noise (Î; blue trace). Vertical gray lines and colored bars identify consecutive time windows over which the mean of I and the variance of Î were calculated. (B) Plot of −σ2(Î)/i as a function of −m(I) (colored circles), calculated for the segments of time shown in A. The solid black line was obtained by linear regression through the data and corresponds to a Po of 0.93. Gray shaded area identifies the region corresponding to Po values >0.8. (C) Current from five TRPM2 channels recorded in the continuous presence of 398 µM Ca2+ plus 32 µM ADPR. Red arrows mark the time points of irreversible inactivation of the individual channels. Colored bars identify time windows with constant N (between two red arrows). (D) Stability plots of Po, mean open time (m.o.t.), and mean closed time (m.c.t.) for the five individual time windows with constant N, identified by color coding in C. Values were obtained by the cycle-time method (see section 4 in the online supplemental material, available at http://www.jgp.org/cgi/content/full/jgp.200810109/DC1) and are not corrected for missed events because of the filter dead time. Pipette [Ca2+] was ∼4 µM in both A and C.

Figure 4. Determination of [Ca2+]i dependence of TRPM2 opening and closing rates. (A) Two representative current traces from patches with smaller numbers of TRPM2 channels superfused with various test [Ca2+]i (bars) in the presence of 32 µM ADPR. Pipette [Ca2+] was ∼4 µM. (left) The number of channels (N) in test segments at low micromolar Ca2+ (blue bars; expanded below) was obtained by linear interpolation of N in bracketing segments at saturating Ca2+. (right) Test segments for [Ca2+]i >40 µM were defined as the time periods between two occurrences of irreversible channel closure (red arrows). Within such segments (blue bars; expanded below) Po approached unity and N was given by the maximum current level. (B) Closing rate at various submicromolar [Ca2+]i was studied in a macropatch; in the presence of 32 µM ADPR TRPM2 channels were alternately exposed to 125 µM Ca2+ and various submicromolar test [Ca2+] (bars); pipette [Ca2+] was ∼4 µM. Current decay time courses in various test [Ca2+]i were fitted by single exponentials (colored smooth lines and time constants [in milliseconds]); those in 8 nM (blue), 300 nM (black), and 4.4 µM Ca2+ (red) are shown below at an expanded time scale. Note the complete lack of reopening events in 8 and 300 nM Ca2+; in 4.4 µM Ca2+ opening rate is still far smaller than closing rate as witnessed by the small remaining steady-state current.

Figure 6. Closure of open channels is slowed by millimolar extracellular Ca2+. (A) Representative time courses of macroscopic current decay upon sudden removal of intracellular Ca2+, with ∼4 µM (left trace) and 1 mM (right trace) free Ca2+ in the pipette solution. Smooth lines are single-exponential fits, with time constants shown. Inset shows mean ± SEM decay time constants for the above two conditions. (B) Cartoon interpretation of channel closing kinetics when intracellular Ca2+ is washed away. (top) In the absence of extracellular Ca2+ the activating sites rapidly lose Ca2+ yielding unliganded channels that close fast (Fig. 5 D, solid arrow). (bottom) In the presence of extracellular Ca2+ the activating sites, which are located in a deep vestibule near the pore entrance, remain liganded because of Ca2+ ions entering through the open pore. Thus, channels close at the slow rate characteristic of fully liganded channels (Fig. 5 D, dotted arrow). Once channels have closed, the activating sites, which are located intracellularly of the gate, are cut away from Ca2+, hence channels remain shut. Note a few occasional reopening events in the right current trace in A, which typically follow brief closures (e.g., blue arrow), suggesting that some fraction of very brief closed events is too short to allow dissociation of Ca2+ from the activating sites. Such occasional reopenings might explain why in the presence of 1 mM of extracellular Ca2+ and 4.4 µM [Ca2+]i the macroscopic current relaxations yield a slightly smaller estimate of closing rate than the steady-state data (Fig. 5 C, white circle vs. diamond for 4.4 µM [Ca2+]i).

Figure 7. Depolarization removes the gating effect of extracellular Ca2+ by preventing Ca2+ influx through the open pore. (A and B) In the presence of 32 µM ADPR macroscopic TRPM2 currents at various test potentials (bars below traces) were activated by exposure to 125 µM of intracellular Ca2+ and closed by its sudden removal. Colored smooth lines are fitted single exponentials and time constants are in milliseconds. Pipette [Ca2+] was ∼4 µM in A and 1 mM in B. (C) Closing rates (mean ± SEM) in zero [Ca2+]i and ∼4 µM (black circles) or 1 mM (red circles)of free Ca2+ in the pipette solution, obtained as the reciprocals of current decay time constants (see A and B, respectively), are plotted against membrane potential.